Specifies the action to take when the stencil test fails.
Six symbolic constants are accepted: GL_KEEP, GL_ZERO, GL_REPLACE, GL_INCR,
GL_DECR, and GL_INVERT. The initial value is GL_KEEP.

zfail

Specifies the
stencil action when the stencil test passes, but the depth test fails. zfail
accepts the same symbolic constants as fail. The initial value is GL_KEEP.

zpass

Specifies the stencil action when both the stencil test and the depth
test pass, or when the stencil test passes and either there is no depth
buffer or depth testing is not enabled. zpass accepts the same symbolic
constants as fail. The initial value is GL_KEEP.

Stenciling, like
depth-buffering, enables and disables drawing on a per-pixel basis. You draw
into the stencil planes using GL drawing primitives, then render geometry
and images, using the stencil planes to mask out portions of the screen.
Stenciling is typically used in multipass rendering algorithms to achieve
special effects, such as decals, outlining, and constructive solid geometry
rendering.

The stencil test conditionally eliminates a pixel based on the
outcome of a comparison between the value in the stencil buffer and a reference
value. To enable and disable the test, call glEnable and glDisable with
argument GL_STENCIL_TEST; to control it, call glStencilFunc.

glStencilOp
takes three arguments that indicate what happens to the stored stencil
value while stenciling is enabled. If the stencil test fails, no change
is made to the pixel's color or depth buffers, and fail specifies what happens
to the stencil buffer contents. The following six actions are possible.

GL_KEEP

Keeps the current value.

GL_ZERO

Sets the stencil buffer value to 0.

GL_REPLACE

Sets the stencil buffer value to ref, as specified by glStencilFunc.

GL_INCR

Increments the current stencil buffer value. Clamps to the maximum representable
unsigned value.

GL_DECR

Decrements the current stencil buffer value. Clamps
to 0.

GL_INVERT

Bitwise inverts the current stencil buffer value.

Stencil
buffer values are treated as unsigned integers. When incremented and decremented,
values are clamped to 0 and $2 sup n - 1$, where $n$ is the value returned
by querying GL_STENCIL_BITS.

The other two arguments to glStencilOp specify
stencil buffer actions that depend on whether subsequent depth buffer tests
succeed (zpass) or fail (zfail) (see glDepthFunc). The actions are specified using the same six symbolic constants
as fail. Note that zfail is ignored when there is no depth buffer, or when
the depth buffer is not enabled. In these cases, fail and zpass specify
stencil action when the stencil test fails and passes, respectively.